Recently, the drivers of automobiles have directed their attention to air-bag safety systems, which protect them during unexpected collisions. One of the important technologies in the air-bag safety system is an acceleration sensor, which detects the occurance of a collision. In early years, a mechanical acceleration sensor was used, but its structure was not simple; its size and a weight were not small; its cost was high, and its response was not fast. Subsequently, engineers of the air-bag safety system placed their hopes on a semiconductor acceleration sensor. The essential portion of the semiconductor acceleration sensor comprises a weight portion, which is supported by beams at the right and left edges thereof, and the vector of the acceleration of the weight portion is perpendicular to the beams. It can be seen that an inertial force acting on the weight portion causes bending stresses in the beams, and thereby piezo-electric resistances are caused in the beams, from which the value of the acceleration of the weight portion can be detected. In the semiconductor acceleration sensor, clearances are formed around the weight portion, in order to assure the free movement of the weight portion.
Although almost all disadvantages of the mechanical acceleration sensor are swept away by the semiconductor acceleration sensor, many other difficulties arise in this sensor. One problem is that when foreign bodies get into the clearances, movement of the weight portion is largely disturbed. Another problem is that overcoming the aforementioned difficulties only by a high precision machining process requires a large amount of investment, and this way will not be adopted from an economical view point.